The non-volatile memory such as a flash memory and a SONOS memory, which has currently become the mainstream of the market, uses such a technique as electric charges accumulated in an insulating film provided above a channel portion cause threshold voltage of a semiconductor transistor to be changed. Microfabrication is required to provide larger capacity, but even a simple semiconductor transistor having no charge accumulation function has become difficult to microfabricate. With that in mind, a functional division has been considered such that the transistor serves only as a switching function of selecting a memory cell for reading and writing information and the information storage element is separated from the transistor like a DRAM; and each of the transistor and the information storage element undergoes microfabrication separately to provide larger capacity.
As a technique for implementing microfabrication of the information storage element, there has been actively developed a variable resistance element which is an electronic element allowing a value of the electrical resistance to be changed between two or more values by an electrical impulse. In an information storage device accumulating electrical charges in a capacitor, such as a DRAM, signal voltage is lowered with a reduction in the amount of accumulated electrical charges due to microfabrication. But, the variable resistance element is advantageous for microfabrication in that electrical resistance generally holds a finite value with microfabrication, provided that there are a principle of changing resistance and a material having the principle.
The operation of such a variable resistance element serves as a switch switching between the ON state and the OFF state. In principle, the variable resistance element can be applied to, for example, a switch (selector) in a wiring configuration in an LSI.
There have been several techniques for changing electrical resistance by an electrical impulse. The most studied techniques of them include a memory device using a fact that when a pulse current is applied to a chalcogenide semiconductor, the crystal phase states (amorphous state and crystalline state) are changed; the states differ from each other in electrical resistance by 2 or 3 orders of magnitude. Such a memory device is generally called a phase change memory.
On another front, regarding a metal/metal oxide/metal (hereinafter referred to as an MIM type) structure where electrodes sandwich a metal oxide, there has been known that the resistance of the structure can be changed by applying a large voltage or current. In the 1950s to 1960s, there have been studies on various materials exhibiting the phenomenon that resistance varies due to voltage or current. For example, a variable resistance element using nickel oxide (NiO) has been reported in Non-Patent Document 1 (Solid-State Electronics, 1964, vol. 7, pp. 785-797). In general, the phase change memory not only involves a large change in volume with a change in crystal phase but also needs to be heated locally to several 100° C. in a short time of several 10 nsec to change the crystal phase. On the other hand, the MIM type variable resistance element has recently been gaining attention again since the need to heat to a high temperature of several 100° C. has not been reported.
FIG. 1 shows a schematic sectional view for explaining a basic structure of an MIM type variable resistance element; and FIG. 2 shows a current voltage characteristic of the MIM type variable resistance element using a variable resistance material made of a nickel oxide.
The MIM type variable resistance element illustrated in FIG. 1 includes first upper electrode 1, second lower electrode 3, and variable resistance material 2 made of a metal oxide provided therebetween. This variable resistance element can non-volatilely maintain the characteristic of a high resistance OFF state or a low resistance ON state even when the power is turned off. Further, the resistance states thereof can be switched by applying a predetermined voltage/current impulse as needed. When voltage Vt1 or higher is applied to the element in a high resistance OFF state illustrated in FIG. 2(a), the state of the element is changed to a low resistance ON state illustrated in FIG. 2(b). Afterward, when voltage Vt2 or higher is applied to the element in the ON state illustrated in FIG. 2(b), the state of the element is changed to the high resistance OFF state, returning to the electrical characteristic of FIG. 2(a). The MIM type variable resistance element can repeat a switching operation between the state of FIG. 2(a) and the state of FIG. 2(b). This characteristic can be used as a non-volatile memory cell or a non-volatile switch for switching circuits.
Such an MIM type variable resistance element uses a transition metal oxide as a main variable resistance material, and the metal oxide material generally has a high resistivity. It is hence easy for the MIM type variable resistance element to maintain the high resistance state (OFF state as the switch), and therefore it is important to control the transitional process to the low resistance state (ON state as the switch).
The current path for maintaining the low resistance state of the MIM type variable resistance element is not formed over the entire electrode surface, but is formed locally to be approximately several nm, at most several 10 nm in diameter. This current path is schematically illustrated in FIG. 3. In FIG. 3, tubular path 4 connecting first upper electrode 1 and second lower electrode 3 is formed in variable resistance material 2.
FIG. 4 illustrates an electrode area dependence of a resistance in a low resistance state of a parallel plate element configured such that NiO is used as the variable resistance material in the same manner as described in Non-Patent Document 1 described above and the material is sandwiched between the electrodes. FIG. 4 indicates that the resistance in the low resistance state practically does not depend on the electrode area, and clearly demonstrates that the low resistance state is maintained by the locally formed current path.
FIG. 5 is an equivalent circuit diagram in the ON state. RON denotes a resistance corresponding to current path 4 in the ON state. ROFF denotes a state in which current path 4 is not formed, namely, an OFF state resistance. In general, ROFF/RON can be considered to be maintained at about 102 or more.